As interest in environmental problems has been growing in recent years, much importance has been attached to countermeasures against exhaust gases from internal combustion engines such as automotive engines. If exhaust gases from internal combustion engines are emitted into the air as they are, problems such as pollution and environmental deterioration occur. Therefore, the exhaust gases are emitted into the air after purified by using exhaust gas purifying catalysts or the like.
Exhaust gas purifying catalysts are widely employed because they can achieve a drastic reduction in emissions without giving a burden to internal combustion engines such as engines which emit exhaust gases, when compared to other purifying methods.
The exhaust gas purifying catalysts are each produced by forming a porous carrier layer comprising a heat-resistant inorganic oxide or the like on a surface of a heat-resistant catalyst support substrate, and loading catalytic metal on this carrier layer. The exhaust gas purifying catalysts convert harmful nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gases into harmless nitrogen, carbon dioxide and water under the action of the catalytic metal.
In recent years, exhaust gas purifying catalysts each formed by providing different characteristics to one exhaust gas purifying catalyst have been developed for the purposes of installability on a vehicle, cost reduction and so on.
Japanese Unexamined Patent Publication No. H8-114116 discloses a catalytic converter in which a low-oxygen activity zone is provided on an upstream side and a high-oxygen activity zone is provided on a downstream side. This catalytic converter comprises at least one of Pt, Pd and Rh as a catalytic metal and is formed so that the amount of this catalytic metal loaded is different between the low-oxygen activity zone and the high-oxygen activity zone.
Japanese Unexamined Patent Publication No. S61-78438 discloses an exhaust gas purifying monolithic catalyst comprising a catalyst layer having a carrier layer loaded on a monolithic catalyst support and a catalytic metal, wherein an inlet port side of the catalyst layer mainly comprises Pt and an outlet port side of the catalyst layer mainly comprises Pd. This exhaust gas purifying monolithic catalyst is produced by forming a catalyst carrier layer such as an alumina layer on a monolithic catalyst support and immersing this monolithic catalyst support formed with the catalyst carrier layer in an aqueous Pt solution from an end portion on an inlet port side and in an aqueous Pd solution from an end portion on an outlet port side, and burning it, thereby forming a catalyst layer.
Japanese Unexamined Patent Publication No. S62-68544 discloses an exhaust gas purifying monolithic catalyst comprising catalyst layers loaded on a monolithic catalyst support, wherein the catalyst layer on an inlet port side comprises an alumina layer containing Pt and Ce and the catalyst layer on an outlet port side comprises an alumina layer containing Pd and La and/or Nd.
Japanese Unexamined Patent Publication No. H4-276113 discloses a honeycomb catalyst on which a NOx reduction catalyst is loaded on an upstream side and a HC and CO oxidation catalyst is loaded on a downstream side with respect to the flow direction of exhaust gases.
In each of these two exhaust gas purifying catalysts, the carrier layers loaded with catalytic metals are formed of different materials. Namely, in each of these two exhaust gas purifying catalysts, the catalyst layers are formed by preparing slurries having different compositions, coating the slurries respectively for predetermined coating lengths from both end portions of a monolithic support catalyst, and burning the resultant.
As mentioned above, each of the exhaust gas purifying catalysts formed by providing different characteristics on one catalyst support substrate is produced by coating two kinds of coating fluids comprising slurries or catalytic metal solutions and respectively exhibiting different characteristics from both end portion sides of a catalyst support substrate such as a monolithic support. In actually producing an exhaust gas purifying catalyst, when a coating fluid is coated from one end portion side and then a different coating fluid is coated from the other end portion side the coating is carried out so that the coating fluids partially overlap each other. The coating method in which the coating fluids partially overlap each other is carried out in order to suppress exhaust gas purifying performance deterioration due to formation of a portion not coated with any coating fluids.
The conventional exhaust gas purifying catalysts have had a problem of incurring a loss of coating fluids at a coating fluid overlapping portion. The conventional catalysts also have had a problem of being unable to exhibit exhaust gas purifying performance as designed, because the coating fluids prepared so as to exhibit different characteristics overlap each other at the coating fluid overlapping portion. For example, in each of the exhaust gas purifying catalysts disclosed in Japanese Unexamined Patent Publication Nos. H8-114116 and S61-78438, different amounts of catalytic metal are loaded on one carrier layer, but when a portion loaded with a small amount of the catalytic metal and a portion loaded with a large amount of the catalytic metal overlap each other, the amount of the catalytic metal loaded at the overlapping portion increases above a design value.
Furthermore, the conventional catalysts have had a problem that when coating fluids are slurries for forming carrier layers, the thickness of the carrier layers at the coating fluid overlapping portion becomes large and exhaust gas passages become narrow. If the exhaust gas passages become narrow, pressure loss in passing exhaust gases becomes high and a burden is applied on an engine.
Namely, the conventional exhaust gas purifying catalysts have had problems of incurring a loss of materials constituting the coating fluids and lowering characteristics such as exhaust gas purifying performance and airflow performance below design values.